a) Field of the Invention
The present invention relates to a solid-state image pickup device of metal oxide semiconductor (MOS) type and a digital camera using the same for producing mobile picture data and still picture data.
b) Description of the Related Art
Since the mass-producing technique of a charge-coupled device (CCD) has been developed, devices and apparatuses using a CCD-type solid-state image pickup device as a line sensor or an area image sensor have been rapidly spread in the world.
On the other hand, with spread of personal digital assistants and the like, a MOS-type solid-state image pickup device consuming less power than the CCD-type solid-state image pickup device is being developed. In the MOS-type solid-state image pickup device, the power consumption can be lowered to, for example, about from ⅕ to about 1/10 of that of the CCD-type solid-state image pickup device.
A MOS-type solid-state image pickup device includes a semiconductor substrate, a large number of pixels formed in one surface of the semiconductor in the form of a matrix with a plurality of rows and a plurality of columns, an output signal line disposed for each pixel column in the vicinity thereof, and an image signal outputting device electrically connected to the output signal lines. Each pixel includes a photoelectric converter element and at least one transistor electrically connected to the photoelectric converter element. The output signal lines have desirably low resistance and are usually formed by using metallic material.
In this specification, in the array of the pixels arranged in rows and columns, pixels arranged in a direction in which the output signal lines extend form “pixel columns”, and the direction is called “pixel column direction”. A direction intersecting the pixel column direction is called “pixel row direction”, pixels arranged in this direction form “pixel rows”. When the output signal line meanders, a direction in which the output signal line generally extends is assumed as the extending direction of the output signal line.
Several configurations of MOS-type solid-state image pickup devices are known. In one of the known configurations of the MOS-type solid-state image pickup device, each pixel includes an output transistor and a reset transistor. In this specification, the MOS-type solid-state image pickup device of this type is referred to as “MOS-type solid-state image pickup device I” depending on cases.
In the pixel of the MOS-type solid-state image pickup device I, a control terminal (gate electrode) of the output transistor is electrically connected to the photoelectric converter element, and each output signal line has a load resistor. When a voltage representing electric charge accumulated in the photoelectric converter element is applied to the control terminal of the output transistor, an output signal (analog voltage signal) appears or is generated on an associated output signal line. The image signal outputting device outputs the analog voltage signal directly to an external device or converts the analog voltage signal into a digital signal and then outputs the digital signal to an external device.
The-electric charge in the photoelectric converter element is kept remained. Before a next output signal is to be generated on the output signal line, it is necessary after the signal output operation of the photoelectric converter element to discharge the charge remaining in the photoelectric converter element to predetermined wiring line or the like. The reset transistor is used to control the discharge of electric charge from the photoelectric converter element. One reset transistor is arranged for each photoelectric converter element.
In a digital camera having a mobile picture mode to produce mobile picture data and a still picture mode to produce still picture data, the mobile picture mode is ordinarily booted and used in its shooting operation. Only when necessary, the still picture mode is used for the shooting. In the shooting operation, an image signal readout period and a vertical blanking period are alternately set in the digital camera.
In a digital camera including a MOS-type solid-state image pickup device, in each image signal readout period, the MOS-type solid-state image pickup device produces output signals necessary to obtain one frame of image data.
When the MOS-type solid-state image pickup device is a MOS-type, solid-state image pickup device I, the output signals necessary to obtain one frame of image data are sequentially generated on the output signal lines, for example, in the pixel row unit. Operation of each output transistor is controlled in the pixel row unit. One row selection signal line is disposed for each pixel row. A row selection signal to control generation of an output signal is supplied via the row selection signal line to each associated pixel.
In this specification, an operation in which the output signals necessary to obtain one frame of image data are sequentially generated on the output signal lines for each pixel row is called “image signal readout operation”. During one image signal readout period, one image signal readout operation is conducted. Ordinarily, the image signal readout operation starts at a start point of the image signal readout period and ends an end point thereof. One image signal readout period ranges, for example, from about 1/60 second to 1/30 second.
When the MOS-type solid-state image pickup device is a MOS-type solid-state image pickup device I, by sequentially operating the reset transistors in the pixel row unit, a row reset operation can be conducted to sequentially discharge the electric charge accumulated in each photoelectric converter element from the respective elements in the pixel row unit.
To determine a period of exposure time for each photoelectric converter element, the row reset operation is conducted at a predetermined point of time. The row rest operation corresponds to operation of an electronic shutter. If light continuously enters the photoelectric converter elements, charge accumulation sequentially starts again beginning at the pixel row having finished the electronic shutter operation. For each pixel, a period of time from when the electronic shutter operation is finished to when a subsequent image signal readout operation is started is the period of exposure time.
A length of a period of time required to conduct the electronic shutter operation for all pixel rows is substantially equal to a length of one image signal readout period. One electronic shutter operation starts at a predetermined point of time during one image signal readout period and ends at a predetermined point of time during a subsequent image signal readout period.
To control each reset transistor, one reset signal supply line is disposed for each pixel row. A reset signal is supplied via the reset signal line to the associated reset transistor.
When necessary, at a particular point of time during a period of time between an image signal readout operation and a subsequent electronic shutter operation, for example, each time an output signal is generated on each output signal line in the pixel row unit, electric charge is discharged to a predetermined line from the pertinent photoelectric converter elements from which output signals are just outputted. The operation to generate an output signal on each output signal line for each pixel row and the operation to discharge electric charge from the photoelectric converter elements in the pixel row unit are sequentially conducted in this order in the pixel row unit. At a predetermined point of time thereafter, an electronic shutter operation is again conducted.
To supply a row selection signal to each of the row selection signal lines at predetermined timing, a readout row-shifter is usually formed on the pertinent semiconductor substrate. To supply a reset signal to each of the reset signal lines at predetermined timing, a reset row-shifter is usually formed on the pertinent semiconductor substrate.
To produce a digital output signal, an image signal outputting device can be composed of an analog-to-digital (A/D) converter. The A/D converter receives an analog voltage signal and outputs a digital signal representing the analog voltage signal to a buffer memory. In a MOS-type solid-state image pickup device including such an A/D converter, the digital signal from the A/D converter is an image signal.
Operations of each shifter and the image signal outputting device are controlled by a controller. The controller is usually formed on the pertinent semiconductor substrate.
To produce a bright image of an object by shooting the object against backlight, strobe light or flashlight is desirably used to provide sufficient exposure for the object. In other words, backlight correction is desired.
However, there has not been materialized a digital camera which is a digital camera using a MOS-type solid-state image pickup device conducting a row-sequential image signal readout operation and which has a backlight correcting function.
When an object moving at a high speed is shot by using the digital camera, the shutter time varies between an upper section and a lower section of an image of the object, and hence a blur appears in the image. Because, at electronic shutter operation, electric charge accumulated in the photoelectric converter elements is sequentially discharged in the pixel row unit, a point of shutter time varies between the pixel rows.